practical usage of /etc/networks file

Question

What is the practical usage of /etc/networks file? As I understand, one can give names to networks in this file. For example:

root@fw-test:~# cat /etc/networks
default         0.0.0.0
loopback        127.0.0.0
link-local      169.254.0.0
google-dns      8.8.4.4
root@fw-test:~# 

However, if I try to use this network name for example in ip utility, it does not work:

root@fw-test:~# ip route add google-dns via 104.236.63.1 dev eth0
Error: an inet prefix is expected rather than "google-dns".
root@fw-test:~# ip route add 8.8.4.4 via 104.236.64.1 dev eth0
root@fw-test:~#

What is the practical usage of /etc/networks file?

Solution 1

As written in the manual page, the /etc/networks file is to describe symbolic names for networks. With network, it is meant the network address with tailing .0 at the end. Only simple Class A, B or C networks are supported.

In your example the google-dns entry is wrong. It's not a A,B or C network. It's an ip-address-hostname-relationship therefore it belongs to /etc/hosts. Actually the default entry is also not conformant.

Lets imagine you have an ip address 192.168.1.5 from your corporate network. An entry in /etc/network could then be:

corpname 192.168.1.0

When using utilities like route or netstat, those networks are translated (if you don't suppress resolution with the -n flag). A routing table could then look like:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.1.1     0.0.0.0         UG    0      0        0 eth0
corpname        *               255.255.255.0   U     0      0        0 eth0

Solution 2

The ip command never uses a host name for input either, so your example is hardly relevant. Also you've put a host name into /etc/networks, not a network name!

Entries from /etc/networks are used by tools that try to convert numbers to names, e.g. the (deprecated) route command. Without a suitable entry it shows:

# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.1.254   0.0.0.0         UG    0      0        0 eth0
192.168.0.0     *               255.255.254.0   U     0      0        0 eth0

If I now add a line mylocalnet 192.168.0.0 to /etc/networks:

# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.1.254   0.0.0.0         UG    0      0        0 eth0
mylocalnet      *               255.255.254.0   U     0      0        0 eth0

In practice it's never really used.